The Correlation Between Microstructure And Magnetostriction Of Fe-Ga Alloys

Fe-Ga solid solution(known as Galfenol)has drawn considerable interest as a new type magnetostrictive material due to its lower switching field,better malleability,lower cost,and better temperature stability in comparison with the brittle REFe2 intennetallic compounds.Consequently,Fe-Ga alloys offer a good potential for applications in actuators,torque sensors,underwater scanning sonar and so on.Due to the structural diversity,the magnetostriction of Fe-Ga alloys is very sensitive to thennal history.However,the microstructural origin for this sensitivity has prompted much controversy,remaining as one of the fundamental concern since the discovery of Fe-Ga two decades ago.In this dissertation,we have systematically investigated the microstructure-magnetostriction relation of Fe81Ga19 alloy through placing it at the intermediate state between metastable and equilibrium conditions.X-ray diffraction,transmission electron microscopy,optical microscopy,magnetometry and mechanical tests were performed to characterize the microstructural,magnetic and mechanical properties.Main results are as follows.Microstructural origin for the thermal history sensitivity of magnetostriction in Fe-Ga alloy has been revealed.In comparison with the high temperature quenched one,magnetostriction deterioration happens for the slowly-cooled Fe-Ga alloy,which has been attributed to the long-range ordering of the harmful D03 phase.To reveal the underlying origin,we have investigated the microstructures of Fe81Ga19 polycrystalline samples quenched after slow cooling(at?0.3 K/min)to near and below the decomposition phase boundary(DPB,793 K)between A2 and(A2' +L12)phases.In comparison with the sample quenched at 1273 K(much higher than DPB)or the one quenched near DPB(773 K),magnetostriction reduction is observed for the one quenched far below DPB(673 K).The 1273 K quenched sample is within the so-called K state,containing A2 matrix and harmful DO3 nanoprecipitates.Unlike the one quenched at 773 K that bears single A2 phase,the one quenched at 673 K contains secondary L12 phase with negative magnetostriction and phase transformation induced dislocations located close to the grain boundaries.Simultaneously,weak DO3 ordering of the untransformed bcc matrix is also observed.All these microstructure changes are responsible for the magnetostriction deterioration.Our work provides important information why the magnetostriction of Fe81a19 greatly depends on the thermal history.A strategy for maximizing magnetostriction by quenching the slowly-cooled Fe-Ga alloy near an instable phase boundary has been provided.On the basis of revealing the microstructural origin for the magnetostriction deterioration,we further search for the optimum heat-treating condition to maximize the magnetostriction in polycrystalline Fe81Ga19 alloy.The samples were quenched after slow cooling(?1.3 K/min)to a series of temperatures slightly above or below DBP.In comparison with the sample quenched at 830 K(?37 K above DPB)that has smaller magnetostriction and stronger magnetocrystalline anisotropy,the one quenched at 740 K(?53 K below DPB)has significantly higher magnetostriction and weaker magnetocrystalline anisotropy.Although,the XRD data and TEM characterizations reveal that all the samples have A2 structure,the mechanical tests reveal that quenching at 740 K produces much softer elastic modulus than the other ones.It is the lattice softening that results in the significant magnetostriction enhancement.Such lattice softening originates from the lattice shear nature for the A2(?)L12 transformation at the equilibrium condition.Therefore,the decomposition phase boundary is indeed a "thermodynamically instable phase boundary(IPB)".The scenario for maximizing magnetostriction of Fe-Ga alloys for the present strategy is to avoid the harmful DO3 and L12 phases and to produce a softer lattice through approaching the instable phase boundary.The magnetostriction of Fe-Ga alloy has been further enhanced through placing textured materials near the instable phase boundary.The above-mentioned strategy has been applied to further enhance the magnetostriction of hot-rolled samples,in which the grains have strong texture rather than random for the as-caste state.A comparative investigation has been carried out between two hot-rolled Fe81Ga19 sheets subjected to different heat-treatments,one was quenched at 1273 K after homogenization,the other was quenched after slow cooling(1.3 K/min)to 773 K(20 K below IPB).The 1273 K quenched hot-rolled sheet sample indeed has a larger magnetostriction(?//s=70 ppm)than the as-cast sample(?//=50 ppm),confirming the positive role of grain texture.The magnetostriction in the one quenched near IPB has been drastically enhanced to 120 ppm,71.4%higher than that of the sample quenched at 1273 K.TEM characterizations and further mechanical test reveal that this enhancement originates from the lattice softening of A2 single phase by quenching near IPB.The above results not only reveal the microstructural origin for the thermal history sensitivity in Fe-Ga alloys but also provide a feasible strategy to improve the magnetostrictive performance through approaching an instable phase boundary.